151 Executive summary
Human Effects on Climate '''Human activities are continuing to affect the Earth’s energy''' '''budget by changing the emissions and resulting atmospheric''' '''concentrations of radiatively important gases and aerosols and''' '''by changing land surface properties.''' Previous assessments have already shown through multiple lines of evidence that the climate is changing across our planet, largely as a result of human activities. The most compelling evidence of climate change derives from observations of the atmosphere, land, oceans and cryosphere. Unequivocal evidence from in situ observations and ice core records shows that the atmospheric concentrations of important greenhouse gases such as carbon dioxide (CO₂), methane (CH4), and nitrous oxide (N₂O) have increased over the last few centuries. {[[151.2.2_Key_Concepts_in_Climate_Science|1.2.2]], [[151.2.3 Multiple Lines of Evidence for Climate Change|1.2.3]]} '''The processes affecting climate can exhibit considerable natural''' '''variability. Even in the absence of external forcing, periodic and''' '''chaotic variations on a vast range of spatial and temporal scales''' '''are observed.''' Much of this variability can be represented by simple (e.g., unimodal or power law) distributions, but many components of the climate system also exhibit multiple states—for instance, the glacial– interglacial cycles and certain modes of internal variability such as El Niño-Southern Oscillation (ENSO). Movement between states can occur as a result of natural variability, or in response to external forcing. The relationship among variability, forcing and response reveals the complexity of the dynamics of the climate system: the relationship between forcing and response for some parts of the system seems reasonably linear; in other cases this relationship is much more complex. {[[151.2.2_Key_Concepts_in_Climate_Science|1.2.2]]} Multiple Lines of Evidence for Climate Change '''Global mean surface air temperatures over land and oceans''' '''have increased over the last 100 years.''' Temperature measurements in the oceans show a continuing increase in the heat content of the oceans. Analyses based on measurements of the Earth’s radiative budget suggest a small positive energy imbalance that serves to increase the global heat content of the Earth system. Observations from satellites and ''in situ'' measurements show a trend of significant reductions in the mass balance of most land ice masses and in Arctic sea ice. The oceans’ uptake of CO₂ is having a significant effect on the chemistry of sea water. Paleoclimatic reconstructions have helped place ongoing climate change in the perspective of natural climate variability. {[[151.2.3 Multiple Lines of Evidence for Climate Change|1.2.3]]; Figure 1.3} '''Observations of CO₂ concentrations, globally averaged temperature''' '''and sea level rise are generally well within the range of''' '''the extent of the earlier IPCC projections. The recently observed''' '''increases in CH4 and N₂O concentrations are smaller than those''' '''assumed in the scenarios in the previous assessments.''' Each IPCC assessment has used new projections of future climate change that have become more detailed as the models have become more advanced. Similarly, the scenarios used in the IPCC assessments have themselves changed over time to reflect the state of knowledge. The range of climate projections from model results provided and assessed in the first IPCC assessment in 1990 to those in the 2007 AR4 provides an opportunity to compare the projections with the actually observed changes, thereby examining the deviations of the projections from the observations over time. {[[151.3.1 Global and Regional Surface Temperatures|1.3.1]], [[151.3.2 Greenhouse Gas Concentrations|1.3.2]], [[151.3.4 Climate Change Indicators|1.3.4]]; Figures 1.4, 1.5, 1.6, 1.7, 1.10} '''Climate change, whether driven by natural or human forcing,''' '''can lead to changes in the likelihood of the occurrence or''' '''strength of extreme weather and climate events or both.''' Since the AR4, the observational basis has increased substantially, so that some extremes are now examined over most land areas. Furthermore, more models with higher resolution and a greater number of regional models have been used in the simulations and projections of extremes. {[[151.3.3 Extreme Events|1.3.3]]; Figure 1.9} Treatment of Uncertainties '''For AR5, the three IPCC Working Groups use two metrics to communicate''' '''the degree of certainty in key findings:''' (1) Confidence is a qualitative measure of the validity of a finding, based on the type, amount, quality and consistency of evidence (e.g., data, mechanistic understanding, theory, models, expert judgment) and the degree of agreementIn this Report, the following summary terms are used to describe the available evidence: limited, medium, or robust; and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers: very low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. For a given evidence and agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of agreement are correlated with increasing confidence (see Section 1.4 and Box TS.1 for more details).; and (2) Likelihood provides a quantified measure of uncertainty in a finding expressed probabilistically (e.g., based on statistical analysis of observations or model results, or both, and expert judgement).In this Report, the following terms have been used to indicate the assessed likelihood of an outcome or a result: Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%, Unlikely 0–33%, Very unlikely 0–10%, Exceptionally unlikely 0–1%. Additional terms (Extremely likely: 95–100%, More likely than not >50–100%, and Extremely unlikely 0–5%) may also be used when appropriate. Assessed likelihood is typeset in italics, e.g., very likely (see Section 1.4 and Box TS.1 for more details). {1.4; Figure 1.11} Advances in Measurement and Modelling Capabilities '''Over the last few decades, new observational systems, especially''' '''satellite-based systems, have increased the number of observations''' '''of the Earth’s climate by orders of magnitude.''' Tools to analyse and process these data have been developed or enhanced to cope with this large increase in information, and more climate proxy data have been acquired to improve our knowledge of past changes in climate. Because the Earth’s climate system is characterized on multiple spatial and temporal scales, new observations may reduce the uncertainties surrounding the understanding of short timescale processes quite rapidly. However, processes that occur over longer timescales may require very long observational baselines before much progress can be made. {[[151.5.1 Capabilities of Observations|1.5.1]]; Figure 1.12} '''Increases in computing speed and memory have led to the''' '''development of more sophisticated models that describe physical,''' '''chemical and biological processes in greater detail.''' Modelling strategies have been extended to provide better estimates of the uncertainty in climate change projections. The model comparisons with observations have pushed the analysis and development of the models. The inclusion of ‘long-term’ simulations has allowed incorporation of information from paleoclimate data to inform projections. Within uncertainties associated with reconstructions of past climate variables from proxy record and forcings, paleoclimate information from the Mid Holocene, Last Glacial Maximum, and Last Millennium have been used to test the ability of models to simulate realistically the magnitude and large-scale patterns of past changes. {[[151.5.2 Capabilities in Global Climate Modelling|1.5.2]]; Figures 1.13, 1.14} As part of the process of getting model analyses for a range of alternative images of how the future may unfold, four new scenarios for future emissions of important gases and aerosols have been developed for the AR5, referred to as Representative Concentration Pathways (RCPs). {Box 1.1} Navigation [[151 Executive summary|ES]] [[151.1 Chapter preview|1.1]] [[151.2.1 Setting the Stage for the Assessment|1.2.1]] [[151.2.2 Key Concepts in Climate Science|1.2.2]] [[151.2.3 Multiple Lines of Evidence for Climate Change|1.2.3]] [[151.3 Indicators of Climate Change|1.3]] [[151.3.1 Global and Regional Surface Temperatures|1.3.1]] [[151.3.2 Greenhouse Gas Concentrations|1.3.2]] [[151.3.3 Extreme Events|1.3.3]] [[151.3.4 Climate Change Indicators|1.3.4]] [[151.3.4.1 Sea Level|1.3.4.1]] [[151.3.4.2 Ocean Acidification|1.3.4.2]] [[151.3.4.3 Ice|1.3.4.3]] [[151.4.1 Uncertainty in Environmental Science|1.4.1]] [[151.4.2 Characterizing Uncertainty|1.4.2]] [[151.4.3 Treatment of Uncertainty in IPCC|1.4.3]] [[151.4.4 Uncertainty Treatment in This Assessment|1.4.4]] [[151.5 Advances in Measurement and Modelling Capabilities|1.5]] [[151.5.1 Capabilities of Observations|1.5.1]] [[151.5.2 Capabilities in Global Climate Modelling|1.5.2]] [[151.6 Overview and Road Map to the Rest of the Report|1.6]] [[Box 151.1 Description of Future Scenarios|Box 1]] [[151 Frequently asked questions|FAQ]] [[151 References|Refs]] Notes